This invention pertains to a device for determining the cleanliness of a filter or the like. More in particular, the invention is directed to means to determine when a filter of a pressurized fluid system is so clogged with dirt that the clogged filter might harm the system, by starvation for example.
Filters are commonly utilized in many pressurized fluid systems wherein the fluid circulates between a point of use and a sump, a pump, and similarly processing equipment. The purpose of the filter, as is known, is to keep the fluid clean in order to protect the point of use. Such systems are commonly encountered in internal combustion engines as are found in automobiles and large trucks and off-the-road vehicles, stationary installations, and the like. The invention is useful also in systems other than where the circulating liquid is a lubricating oil, much as a system using gases, air filters, filters for water as are used in swimming pools and liquid fuel systems for vehicles and stationary installations, and prime movers generally; hydraulic systems of all kinds, such as earth moving, industrial and aircraft equipment; air systems of all kinds, positive and negative pressure, vacuum pumps and turbines for example; and all sorts of liquid processing systems that include filtration, such as beverage, food and chemical machinery.
Since at least as early as 1962, in military specification MIL-F-8815, the need for devices to measure the amount of dirt in a filter has been known and in use on military and commercial aircraft hydraulic systems, on the filters in such systems. Such hydraulic system filter indicators operate at relatively high pressure and are quite expensive, which is acceptable in aircraft use, but not for general low pressure internal combustion engine use.
Filter capacity in general use is wasted to a large extent, and it is this problem which is alleviated by the invention. That is, in automotive use, it is a common maintenance and service practive to replace the filter in a lubricating oil system at every 4000 or 5000 miles or so, without regard to the condition of the filter. The motorist is alerted to change the filter more often if he drives in "exceptionally dusty" or the like conditions. These manufacturers' recommendations are a best compromise for it was heretofore impractical to measure directly the condition of the filter and consequently it was replaced more often than was necessary, rather than risk possible damage due to a dirty filter.
Similar thinking applies to large trucks, and other such equipment in which the filters were changed even more frequently because a breakdown or a failure of the filter and the concurrent entry of dirt into the engine could prove extremely costly if engine damage resulted. Thus, much filter capacity was and is wasted as a form of engine "insurance."
Even more importantly, this changing of the filter is almost invariably accompanied by a change of the lubricating oil. In the case of an automobile this can be anywhere from 4 to 6 quarts, but in the case of a large truck the amount of oil wasted could run 6 to 12 gallons. In large fleets and from an environmental protection point of view, this is a large problem indeed.
Filtering systems generally fall into two types; those known as full flow wherein all the fluid at all times passes through the filter element, and those which include some sort of partial by-pass around the filter. Both types often include a suitable relief valve whereby unfiltered fluid is delivered to the point of use in the event the filter clogs with dirt. The invention is equally applicable to both general classes of filtering systems.
Thus the addition of the invention device in a pressurized filtered fluid system permits both maximum use of the filter capacity of the filter, as well as maximum use of the circulative fluid. That is, the circulating fluid and filter are not removed from the system and replaced with much of its useful life and filter capacity unused, simply because it is now the proper time to do so, as has been prior practice, but it is only removed and replaced when the invention indicates that the filter is in fact clogged and it is now precisely the right time to replace the filter and the circulation fluid.
Prior filter condition indicators suffer from a large number of problems, all of which are overcome by the present invention. Many such devices required a temperature correction. This was so because the filter would always "seem" to be dirty when starting up full of a cold viscous oil. That is, at relatively low temperatures such as freezing and below, the oil in internal combustion engines is very dense, and would give a false indication that the filter was clogged. Therefore, some correction for temperature had to be made. In the present invention, no such temperature correction is required.
Another problem in prior filter condition indicators is that they often included fairly complex mechanical interconnections which served various purposes. One of the more important purposes was to provide a snap action and lock into the indicated dirty position. This was required so that when the system is shut down, the indicator would still indicate a dirty condition. If there were no such lock, upon de-pressurization, the filter would become depressurized and the indicator would return to an indicated clean position. Such inter-locks and snap action devices suffered from many disadvantages including the fact that they were relatively complex, they required assembly time, they included spring members which were subject to fatigue and unreliability, they invariably required a manual reset, and the like and the other usual disadvantages of mechanical devices. It is essential that a filter condition indicator, including the invention, remain in the last indicated position when the system is shut down because maintenance checks are almost always made when the vehicle is at rest, such as in a garage, rather than when it is working, such as rolling down a highway. The invention accomplishes the desideratum of holding the last indicated position after shut down with no mechanical inter-connection or interlocks or the like, but in a highly simple technically elegant manner.
Another problem with prior devices was the need to accommodate pressure surges without indicating a dirty condition. That is, if for various reasons the filter should experience a surge in pressure, this could be misinterpreted by the indicator device as a filter clogged condition thereby triggering the dirty signal which would in turn be locked into that position. Thus, some accommodation had to be made to differentiate a pressure surge from a true filter dirty condition. The mentioned device accommodates this problem very easily and well.
Another problem with all prior mechanical devices which include springs is that they cannot be fully responsive to a changing system pressure because a spring can exert a fixed force only. The invention indicator self-compensates for varying system pressures by constantly sending the true pressure on both sides of the filter. In automotive lubricating systems the pressure on the oil system can vary between 10 psi and 65 psi between idle speed and cruising speed. The invention device has a built-in pressure differential operating characteristic wherein the differential balance pressure is a predetermined linear ratio of the operating pressure rather than a fixed differential as in a spring biased device. A spring type device with its fixed spring force to set the indicating pressure or the pressure force differential cannot properly accommodate such changing system pressures and flows. Changes in flow rate have no effect on the invention as it is based on system pressure.